A typical automotive differential includes a compound planetary gear set mounted within a housing. The planetary gear set interconnects a pair of output shafts for rotation in opposite directions with respect to the housing. Engine power rotates the housing about a common axis of rotation shared by the output shafts.
The planetary gear set is generally arranged to permit the output shafts to rotate by equal amounts but in opposite directions with respect to the housing. Accordingly, the housing rotates about the common axis of the output shafts at the average speed of the two output shafts. Drive torque is distributed between the two relatively rotating output shafts in accordance with the efficiency of the planetary set.
Sun gear members of the planetary set, also referred to as "side gears", are coupled to inner ends of the output shafts. Planet gear members of the same set operatively connect the two side gears for rotation in opposite directions. Ordinarily, the sun gear members are bevel gears, and the planet gear members are bevel pinions that mesh with both side gears.
However, such bevel gear planetary gear sets are relatively efficient (i.e., only small torque differences can be supported between the output shafts), and this limits the total amount of drive torque that can be delivered to the drive shafts under uneven traction conditions. For example, if drive wheels coupled to the output shafts have uneven amounts of traction, the total drive torque is limited to a little more than two times the amount of drive torque that can be delivered to the drive wheel having less traction.
A wide variety of differential modifications and alternative differential designs have been proposed to make better use of the total traction available to both drive wheels. For example, spring-loaded clutch packs have been used to provide a predetermined minimum resistance to relative rotation between drive shafts (i.e., differentiation). However, the minimum resistance opposes differentiation even when no drive torque is being delivered to the output shafts and yet may provide too little resistance to differentiation when more drive torque is delivered.
Worm gearing has been used in gear differentials to develop considerable frictional resistance to differentiation as a relatively constant proportion of the total drive torque. This torque proportioning characteristic is more commonly expressed as a "bias ratio" which is a ratio of the respective amounts of torque in the two output shafts. Worm gear differentials, such as the one disclosed in U.S. Pat. No. 2,859,641 to Gleasman, generally exhibit bias ratios of 3.5 to 1 or greater over a wide range of total drive torque.
Parallel-axis gearing, such as spur and helical gearing, has also been used in gear differentials to develop frictional resistance to differentiation as a smaller proportion of the total drive torque. For example, U.S. Pat. No. 3,706,239 to Myers discloses a parallel-axis gear differential of a type that exhibits bias ratios of 2.5 to 1 or less.
However, bias ratios within a range of between 2.5 to 1 and 3.5 to 1 are now preferred for many automotive applications. Bias ratios of at least 2.5 to 1 are preferred for delivering sufficient amounts of additional drive torque under uneven traction conditions. In contrast, bias ratios greater than 3.5 to 1 are generally not needed to compensate for traction differences between drive wheels and can interfere with desired differentiation in turns.
To provide bias ratios within the range of 2.5 to 1 to 3.5 to 1, attempts have been made to reduce the bias ratios of worm gear differentials and to increase the bias ratios of parallel-axis gear differentials. For example, the bias ratios of worm gear differentials have been modestly reduced by increasing the side gear lead angles and by using friction reducing washers at gear end faces.
U.S. Pat. No. 5,169,370 to Dye et al. of the present assignee proposes to increase the bias ratio of a parallel-axis gear differential by dividing the planet gears into two gear portions having opposite hand helix angles that increase axial thrust of the planet gears. Although this approach increases bias ratio, the cost of producing the planet gears is also significantly increased.
An alternative approach to increasing bias ratio in a parallel-axis gear differential is proposed in United Kingdom Patent Application 2,234,791. Pairs of planet gears are distributed asymmetrically about the side gears so that a radial force urges the side gears into a frictional engagement with the housing. This approach limits the size or number of planet gears that can be used to operatively connect the side gears, adds to housing complexity, and subjects the housing to uneven distortions.